(a) Field of the Invention:
The present invention concerns a semiconductor oscillator operated at an extremely-high frequency which is even higher than "super-high", and more particularly it pertains to an oscillator operated at an extremely-high frequency using a seminconductor transit time device having a frequency-dependent negative resistance.
(b) Description of the Prior Art:
Attention of those concerned is being given to an oscillator using--as its semiconductor power producing unit--constituting member for producing oscillation in an extremely high frequency region in terms of microwave, millimeter wave or submillimeter wave--a Gunn diode or an IMPATT (IMPact-ionization Avalanche Transit Time) device, or a TUNNETT (TUNNEl injection Transit Time) diode. Of these known types of diodes, the Gunn diode is a semiconductor device having no pn junction, and this device was noted for the first time by Gunn of IBM to produce current oscillation when operated in the microwave frequency region. Its operation mechanism is such that, when a high voltage is applied across the opposite terminals of a bulk semiconductor plate to elevate its internal electric field intensity E to a value above a certain level, i.e. above a critical (threshold) value, the diode plunges into an electrically unstable state which is called the Gunn effect wherein the change of the average drift velocity v of carries relative to the field intensity E, i.e. differential mobility dV/dE, becomes negative, and the device develops current oscillation.
A solid-state microwave power producing unit using this Gunn diode which serves as an important component of this power producing unit has attracted quite a bit of attention for some time since its "debut". However, this Gunn diode has the drawback that, as the temperature of the device rises during its operation, the drift velocity as well as the mobility of those electrons which are carriers will drop substantially, so that the microwave output power progressively drops with the rise in the device temperature, and that, further, the oscillation frequency threshold peculiar to this diode is low, being only about 100.about.130 GHz at most which is much lower as compared to that of the TUNNETT diode and that of the IMPATT diode, in which oscillation frequencies are both in the level of the submillimeter wave region.
The IMPATT diode and the TUNNETT diode, on the other hand, are called transit time negative resistance devices, in which oscillation frequencies are much higher than that of the Gunn diode, and they have attracted the attention as being the devices having the capability of outputting a high oscillation power even in such a high frequency region as the submillimeter wave region.
Of these two types of diodes, the IMPATT diode is a device having a pn junction which is designed to be operative sothat an avalanche breakdown is caused by a reverse voltage applied across this pn junction, and that this avalanche breakdown, in turn, causes an injection, across the pn junction, of those carriers produced in either contiguous region, and that by virtue of this avalanche breakdown and also of the transit time effect of the carriers thus produced, a frequency-dependent negative resistance caused by a phase delay between the current and the voltage applied leads to the development of an oscillation and an amplification at a high frequency.
While the IMPATT diode features a relatively high output power at a high operating frequency on the one hand, it has the drawback that carriers (electrons and positive holes) are produced as a result of the avalanche breakdown, and these carriers will drift through the bulk, causing the tendency that the device is broken easily. Another important drawback is that the drifting carriers cause very large noises.
Especially in the IMPATT diode, there is the inconvenience that a rise in the temperature of the device brings about a difficulty in causing an injection of carriers by relying on the avalanche breakdown. Hence the drawback that the output power thereof as the oscillator component drops in the frequency region around 100 GHz or thereabove. This is reported also in, for example, Chapter 2, "IMPATT Devices for Generation of Millimeter Waves" by H. J. KUNO, and in "Infrared & Millimeter Waves", Vol. 1, Academic Press, 1979, edited by K. J. Button. It is stated there to the effect that the frequency of oscillation and the power output of an IMPATT diode are strongly dependent upon the junction temperature, and that the typical temperature (T) coefficient of an Si IMPATT diode for frequency drift in the millimeter wave region is -5.times.10.sup.-5 /.degree.C. and that its temperature coefficient for power output (P) variation, i.e. for the power drop rate dP/dT, is -0.005 dB/.degree.C. Thus, the temperature coefficient for power output in an IMPATT diode is usually negative.
It has been said that usually in a Si IMPATT diode, the junction temperature must be kept at a level lower than about 200.degree. C. in order to preserve the electrical characteristics of the device and also to prevent the thermal burnout. In practice, however, since an input power is applied to the diode to cause its oscillation, the junction temperature will naturally rise. Also, even where a heat-sink such as a copper plate is provided on the device to obtain a practical frequency or output power, it is the present state of art to operate the device at a temperature level around the upper limit of the above-mentioned temperature range. As one of the techniques intended to avoid the excessive rise of the temperature, T. ISHIBASHI et al. have reported the technique to effect cooling, by liquid N.sub.2, the IMPATT diode designed to be operated at a high frequency ranging especially from the millimeter wave region to the submillimeter wave range (T. ISHIBASHI, M. INO, T. MAKIMURA and M. OHMORI, "LIQUID NITROGEN-COOLED SUBMILLIMETER-WAVE SILICON IMPATT DIODE", Electronics Letters, 12th May, 1977, vol. 13, No. 10 pp. 299.about.230). This technique, however, has the drawback that the entire device as a solid-state oscillator assumes a large size.
In contrast thereto, the TUNNETT diode is designed to develop a tunnel breakdown in place of the avalanche breakdown which takes place in case of the IMPATT diode, and by virtue of the frequency-dependent negative resistance similar to that noted in the IMPATT diode which, in case of the TUNNETT diode, is brought about by both of said tunnel breakdown and the transit time effect of the carriers produced thereby, there are performed an oscillation and an amplification at a high frequency.
The TUNNETT diode features the injection of carriers from one region having a certain conductivity type into its adjacent region having an opposite conductivity type by virtue of said tunnel breakdown, i.e. the so-called tunnel effect, so that it has the advantage that there arises very little noise.
As will be noted from the foregoing statement, the Si IMPATT diode which has hitherto attracted the attention of those concerned as being a semiconductor junction device which develops oscillation with a relatively high level of output power at the conventionally used highest frequency region has the drawbacks that it develops a large noise during its operation and that, in the frequency region of about 100 GHz or higher, there arises a drift (fluctuation) of an oscillation frequency and also a reduction of the output power due to the rise in the junction temperature, and further it involves the problems that, in case a cooling means using, for example, liquid N.sub.2 is provided, the device as a whole becomes larger in size.
Apart from the above-mentioned semiconductor devices, a vacuum tube, e.g. a backward wave tube (tradename: Carcinotron) provides a large output in the frequency range from millimeter wave region to submillimeter wave region. However, it has the following important drawbacks: that its life time is very short, being about 300 hours, which is very much shorter than that of such a semiconductor device as the IMPATT diode; that it requires an operating voltage as high as 1000 V or greater, thus requiring a very large operating power supply and also a plurality of power supply circuit systems for operating the tube; and that its overall weight as a system is large and also the system is expensive.